1. Field of the Invention
The present invention relates to a method of driving an electronic display device having EL (electro luminescence) elements formed on a substrate. More particularly, the invention relates to a method of driving an EL display device using semiconductor elements (elements using semiconductor thin films) as well as to electronic equipment of the type in which an EL display device is used as a display part.
Incidentally, the term “EL element” used herein indicates both an element which uses emission from a singlet exciter (fluorescence) and an element which uses emission from a triplet exciter (phosphorescence).
2. Description of the Related Art
In recent years, in the field of self-emitting elements, the development of EL display devices having EL elements has been becoming more and more active. EL display devices are called organic EL displays (OELD(s)) or organic light emitting diodes (OLED(s)).
Such an EL display device is of the self-emitting type which differs from liquid crystal devices. An EL element has a structure in which an EL layer is interposed between a pair of electrodes (an anode and a cathode), and ordinary EL layers have a stacked structure. Representatively, there is a stacked structure which is called “hole transport layer/light emitting layer/electron transport layer”, proposed by Tang et al. of Kodak Eastman Company. This structure has very high emission efficiency, and is adopted in nearly all EL display devices currently under research and development.
Other structures may also be adopted, such as a structure in which “a hole injection layer, a hole transport layer, a light emitting layer and an electron transport layer” are stacked on an anode in that order, or a structure in which “a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer and an electron injection layer” are stacked on an anode in that order. The light emitting layer may also be doped with a fluorescent pigment or the like.
All the layers provided between a cathode and an anode are herein generically called “EL layer”. Accordingly, all the aforementioned hole injection layer, hole transport layer, light emitting layer, electron transport layer and electron injection layer are encompassed in the EL layer.
When a predetermined voltage is applied across a pair of electrodes (both electrodes) of the EL layer with the above-described structure, recombination of carriers occur in the emitting layer, whereby the EL element emits light. Incidentally, “EL element emits light” is herein called “EL element is driven”.
As a driving method for the EL display device, there is an active matrix type EL display device.
FIG. 3 shows an example of the construction of a pixel portion of an active matrix type EL display device. A gate signal line (Gl to Gy) to which a selection signal is to be inputted from a gate signal line driver circuit is connected to the gate electrode of a switching TFT 301 which is provided in each pixel of the pixel portion. Either one of the source and drain regions of the switching TFT 301 provided in each pixel is connected to a source signal line (Sl to Sx) to which a signal is to be inputted from a source signal line driver circuit, while the other is connected to the gate electrode of an EL driving TFT 302 and to either one of the electrodes of a capacitor 303 which is provided in each pixel. The other electrode of the capacitor 303 is connected to a power supply line (V1 to Vx). Either one of the source and drain regions of the EL driving TFT 302 provided in each pixel is connected to the power supply line (V1 to Vx), while the other is connected to the other electrode of the EL element 304 provided in each pixel.
The EL element 304 has an anode, a cathode and an EL layer provided between the anode and the cathode. In the case where the anode of the EL element 304 is connected to the source region or the drain region of the EL driving TFT 302, the anode and the cathode of the EL element 304 become a pixel electrode and a counter electrode, respectively. Contrarily, in the case where the cathode of the EL element 304 is connected to the source region or the drain region of the EL driving TFT 302, the cathode and the anode of the EL element 304 become a pixel electrode and a counter electrode, respectively.
Incidentally, the potential of the counter electrode is herein called “counter potential”, and a power source for applying the counter potential to the counter electrode is herein called “counter power source”. The difference between the potential of the pixel electrode and the potential of the counter electrode is an EL driving voltage, and the EL driving voltage is applied to the EL layer.
As a gray scale display method for the above-described EL display device, there are an analog gray scale method and a time gray scale method.
First, the analog gray scale method for the EL display device will be described below. FIG. 4 is a timing chart showing the case where the display device shown in FIG. 3 is driven by the analog gray scale method. The period from the moment when one gate signal is selected until the moment when the next gate signal line is selected is herein called “one line period (L)”. The period from the moment when one image is selected until the moment when the next image is selected corresponds to one frame period. In the case of the EL display device shown in FIG. 3, since the number of gate signal lines is “y”, y-number of line periods (L1 to Ly) are provided in one frame period.
As the resolution of the EL display device becomes higher, the number of line periods for one frame period becomes larger, and the driver circuit of the EL display device must be driven at a higher frequency.
The power source lines (V1 to Vx) are kept at a constant voltage (power source potential). In addition, the counter potential is kept constant. The counter potential has a potential difference from the power source potential to such an extent that the EL elements emit light.
In the first line period (L1), a selection signal from the gate signal line driver circuit is inputted to the gate signal line G1. Then, analog video signals are inputted to the source signal lines (S1 to Sx) in this order.
Since all the switching TFTs 301 connected to the gate signal line G1 are turned on, the analog video signals which have been inputted to the source signal lines (S1 to Sx) are respectively inputted to the EL driving TFTs 302 via the switching TFTs 301.
According to the potential of the analog video signal inputted to each of the pixels when the switching TFT 301 is turned on, the gate voltage of the EL driving TFT 302 varies. At this time, the drain current of the EL driving TFT 302 is determined at a 1-to-1 ratio to the gate voltage thereof in accordance with the Id-Vg characteristic of the EL driving TFT 302. Specifically, according to the potential of the analog video signals inputted to the gate electrode of the EL driving TFT 302, the potential of the drain region of the EL driving TFT 302 (an EL driving voltage corresponding to the on state of the switching TFT 301) is determined and a predetermined drain current flows into the EL element 304, and the EL element 304 emits light at the amount of emission corresponding to the amount of the drain current.
When the above-described operations are repeated until the termination of inputting the analog video signals to the respective source signal lines (S1 to Sx), the first line period (L1) terminates. Incidentally, one line period may also be defined as the sum of the period required until the termination of inputting the analog video signals to the respective source signal lines (S1 to Sx) and a horizontal retrace period. Then, the second line period (L2) starts, and a selection signal is inputted to the gate signal line G2. Similarly to the first line period (L1), analog video signals are inputted to the source signal lines (S1 to Sx) in this order.
When selection signals are inputted to all the gate signal lines (Gl to Gy), all the line periods (L1 to Ly) terminate. When all the line periods (L1 to Ly) terminate, one frame period terminates. During one frame period, all the pixels perform displaying and one image is formed. Incidentally, one frame period may also be defined as the sum of all the line periods (L1 to Ly) and a vertical retrace period.
As described above, the amounts of emissions of the respective EL elements are controlled by the analog video signals, and gray scale display is provided by the control of the amounts of emissions. In this manner, in the analog gray scale method, gray scale display is carried out by the variations in the potentials of the respective analog video signals inputted to the source signal lines.
The time gray scale method will be described below.
In the time gray scale method, digital signals are inputted to pixels to select the emitting states or the non-emitting states of the respective EL elements, whereby gray scales are represented by the cumulation of periods per frame period during which each of the EL elements.
In the following description, 2n gray scales (n is a natural number) are represented. FIG. 5 is a timing chart showing the case where the display device shown in FIG. 3 is driven by the time gray scale method. One frame period is divided into n-number of sub-frame periods (SF1 to SFn). Incidentally, the period for which all the pixels of the pixel portion displays one image is called “one frame period (F)”. Plural periods into which one frame period is divided are called “sub-frame periods”, respectively. As the number of gray scales increases, the number by which one frame period is divided also increases, and the driver circuit of the EL display device must be driven at a higher frequency.
One sub-frame period is divided into a write period (Ta) and a display period (Ts). The write period is the period for which digital signals are inputted to all the pixels during one sub-frame period, and the display period (also called “lighting period”) is the period for which the respective EL display devices assume their emitting states or non-emitting states in accordance with the input digital signals, thereby performing displaying.
The EL driving voltage shown in FIG. 5 represents the EL driving voltage of an EL element for which emitting state is selected. Specifically, the EL driving voltage (FIG. 5) of the EL element for which emitting state is selected is 0 V during the write period, and has, during the display period, a magnitude which enables the EL element to emit light.
The counter potential is controlled by an external switch (not shown) so that the counter potential is kept at approximately the same level as the power source potential during the write period, and has, during the display period, a potential difference from the power source potential to such an extent that the EL element can emit light.
The write period and the display period of each sub-frame period will first be described in detail with reference to FIGS. 3 and 5, and subsequently, the time gray scale method will be described.
First, a gate signal is inputted to the gate signal line G1, and all the switching TFTs 301 connected to the gate signal line G1 are turned on. Then, digital signals are inputted to the source signal lines (S1 to Sx) in that order. The counter potential is kept at the same level as the potential of the power supply lines (V1 to Vx) (power source potential). Each of the digital signals has information of “0” or “1”. Each of the digital signals of “0” or “1” means a signal which has a voltage of high level or low level.
Then, the digital signals which have been inputted to the source signal lines (S1 to Sx) are respectively inputted to the gate electrodes of the EL driving TFTs 302 via the switching TFTs 301 which are in the on state. The respective digital signals are also inputted to the capacitors 303.
Then, the above-described operations are repeated by inputting gate signals to the respective gate signal lines (G2 to Gy), whereby digital signals are inputted to all the pixels and the input digital signal is held in each of the pixels. The period required until the digital signals are inputted to all the pixels is called “write period”.
When the digital signals are inputted to all the pixels, all the switching TFTs 301 are turned off. Thus, an external switch (not shown) connected to the counter electrode causes the counter potential to vary so that a potential difference which enables the EL element 304 to emit light is produced between the counter potential and the power source potential.
In the case where the digital signals have information of “0”, the EL driving TFTs 302 are turned off and the EL elements 304 do not emit light. Contrarily, in the case where the digital signals have information of “1”, the EL driving TFTs 302 are turned on. Consequently, the pixel electrodes of the respective EL elements 304 are kept at approximately the same potential as the power source potential, and the EL elements 304 emit light. In this manner, the emitting states or the non-emitting states of the EL elements 304 are selected in accordance with the information of the digital signals, and all the pixels perform displaying at the same time. When all the pixels perform display, an image is formed. The period for which the pixels perform displaying is called “display period”.
The lengths of the write periods (Ta1 to Tan) of all the n-number of sub-frame periods (SF1 to SFn) are the same. The display periods (Ts) of the respective sub-frame periods (SF1 to SFn) are denoted by Ts1 to Tsn.
The lengths of the respective display periods are set to become Ts1:Ts2:Ts3: . . . :Ts(n−1):Tsn=20:2−1:22: . . . :2−(n−2):2−(n−1), respectively. By combining the desired ones of these display periods, it is possible to provide display in the desired number of gray scales within 2n gray scales.
The display period is any one of Ts1 to Tsn. Here, it is assumed that predetermined pixels are turned on for the period of Ts1.
Then, when the next write period starts and data signals are inputted to all the pixels, the next display period starts. At this time, the display period is any one of Ts2 to Tsn. Here, it is assumed that predetermined pixels are turned on for the period of Ts2.
It is assumed that the same operations are repeated as to the remaining (n−2)-number of sub-frames, whereby the display periods are set as Ts3, Ts4, . . . , Tsn in this order and predetermined pixels are turned on during each of the sub-frames.
When the n-number of sub-frame periods appear, one frame period terminates. At this time, the gray scale of a pixel is determined by cumulatively calculating the length of the display period for which the pixel has been turned on. For example; assuming that n=8 and the obtainable luminance in the case where the pixel emits light for all the display period is 100%, if the pixel emits light during Ts1 and Ts2, a luminance of 75% can be represented, and if Ts3, Ts5 and Ts8 are selected, a luminance of 16% can be realized.
Incidentally, in the driving method using the time gray scale method which represents gray scales by inputting n-bit digital signals, the number of plural sub-frame periods into which one frame period is divided, the lengths of the respective sub-frame periods and the like are not limited to the above-described examples.
The above-described analog gray scale method has problems to be described below.
The analog gray scale method has the problem that the unevenness of the characteristics of TFTs greatly affects gray scale display. For example, it is assumed that the Id-Vg characteristics of switching TFTs differ between two pixels which represent the same gray scale (the characteristic of either one of the pixels is shifted as a whole to a plus or minus side relative to the characteristic of the other).
In this case, the drain currents of the respective switching TFTs take different values, and gate voltages with different values are applied to the EL driving TFTs of the respective pixels. In other words, different amounts of currents flow into the EL elements of the respective pixels, and as a result, the amounts of emissions from the EL elements differ from each other and the same gray scale cannot be represented.
Even if equal gate voltages are applied to the EL driving TFTs of the respective pixels, the EL driving TFTs cannot output the same amount of drain current so long as the Id-Vg characteristics of the EL driving TFTs are not even. For this reason, if the Id-Vg characteristics of the switching TFTs slightly differ from each other, the amounts of currents outputted from the EL driving TFTs greatly differ from each other even when equal gate voltages are applied to the EL driving TFTs. As a result, owing to a slight unevenness of the Id-Vg characteristics, the amounts of emissions from the EL elements greatly differ between adjacent pixels even if signals of the same voltage are applied to the EL driving TFTs.
Gray scale display actually becomes far more non-uniform owing to a synergistic effect of the unevenness of the characteristics of the switching TFTs and the unevenness of the characteristics of the EL driving TFTs. Thus, analog gray scale display is extremely sensitive to the unevenness of the characteristics of TFTs. Accordingly, when this EL display device provides gray scale display, there is the problem that the display becomes considerably uneven.
The time gray scale method has a problem to be described below.
In the time gray scale method, the luminance of an EL element is represented by the time for which a current flows in the EL element and the EL element emits light. Accordingly, it is possible to greatly suppress the non-uniformity of display due to the unevenness of the characteristics of TFTs, which is a problem in the analog gray scale method. However, there is another problem.
The current which flows in the EL element is controlled by a voltage to be applied across both electrodes of the EL element (EL driving voltage). This EL driving voltage is a voltage obtained by subtracting the voltage across the drain and the source of an EL driving TFT from the potential difference between a power source potential and a counter potential. In order to avoid the influence of the non-uniformity of drain-source voltages due to the unevenness of the characteristics of EL driving TFTs and keep the EL driving voltage constant, the voltage across the drain and the source of the EL driving TFT is set to be far smaller than the EL driving voltage. At this time, the EL driving TFT is operating in a linear region.
In the TFT operation, the linear region corresponds to the operating region in which a voltage VDS across the drain and the source of the TFT is smaller than a gate voltage VGS of the TFT.
Here, the current flowing between both electrodes of the EL element is influenced by temperature. FIG. 17 is a graph showing the temperature characteristic of the EL element. From this graph, it is possible to know the amounts of currents which flow between both electrodes of the EL element with respect to voltages applied across both electrodes of the EL element at certain temperatures. A temperature T1 is higher than a temperature T2, and the temperature T2 is higher than a temperature T3. As can be seen from FIG. 17, even if the voltage applied across the both electrodes of the EL element in the pixel portion is the same, the current flowing between both electrodes of the EL element becomes larger owing to the temperature characteristic of the EL element as the temperature of the EL element becomes higher.
The luminance of the EL element is proportional to the amount of current flowing between both electrodes of the EL element.
In this manner, the time gray scale method has the problem that the current flowing between both electrodes of the EL element varies owing to variations in the environmental temperature at which the EL display device is used if a constant voltage is continuously applied across both electrodes of the EL element, and the luminance of the EL display device varies and accurate gray scale display becomes impossible.
In the active matrix type EL display device, for the above-described reasons, if the conventional analog gray scale method or time gray scale method is used, it is impossible to perform accurate gray scale display.